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68

Information Notice No. 93­68: Failure of Pump Shaft Coupling Caused

by Temper Embrittlement During Manufacture

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555

September 1, 1993

INFORMATION NOTICE 93‐68: FAILURE OF PUMP SHAFT COUPLING CAUSED BY

TEMPER EMBRITTLEMENT DURING MANUFACTURE

Addressees

All holders of operating licenses or construction permits for nuclear power

reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information

notice to alert addressees to problems caused by temper embrittlement of

American Iron and Steel Institute Type 410 stainless steel couplings supplied

by Byron Jackson. It is expected that recipients will review the information

for applicability to their facilities and consider actions, as appropriate, to

avoid similar problems. However, suggestions contained in this information

notice do not constitute NRC requirements; therefore, no specific action or

written response is required.

Description of Circumstances

On June 20, 1991, a river water pump shaft coupling at Beaver Valley Nuclear

Power Plant, Unit 1, failed during operation when a large section of one end

of the coupling broke away from the rest of the coupling. This coupling,

which was threaded internally, was used to join two shafts of a Byron Jackson

vertical circulator river water pump. During its investigation of the

failure, the licensee found that two more couplings from the same pump (river

water pump 1A) had cracks. All three of the Unit 1 pump shafts had at least

one of the defective couplings. The licensee at Beaver Valley noted that

increased vibration levels on pump 1A caused by a worn bearing, pump shaft

misalignment or both contributed to the failure.

All of these couplings were part of a group of 13 couplings that had been

acquired in 1977 from Byron Jackson under the same purchase order. However,

the licensee was only able to locate six of the couplings for examination and

surmised that the rest were used and had been disposed of during routine

maintenance. All of these couplings had been fabricated from the same heat

treatment batch of Type 410 stainless steel (steel heat number HT821336).

Metallurgists at Lehigh University conducted a root cause analysis of the

damaged Unit 1 couplings and concluded that failure was caused by the low
impact strength of the couplings due to temper embrittlement resulting from

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IN 93‐68

September 1, 1993

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improper heat treatment. They also concluded that all of the couplings from

this heat of steel were temper embrittled. Under NRC contract, Brookhaven

National Laboratory conducted a detailed metallurgical failure analysis of the

broken coupling taken from pump 1A. Brookhaven similarly concluded that the

failure was caused by temper embrittlement of the Type 410 stainless steel

brought about by improper heat treatment of the coupling during manufacture.

In 1992, three couplings in Beaver Valley Unit 2 river water pumps were found

to have cracks: two in pump 2A and one in pump 2C. These pumps together with

the couplings were also supplied by Byron Jackson. The Unit 2 couplings are

keyed sleeves instead of threaded couplings. The cracks were located in the

corners of the keyway slots. Lehigh University metallurgists examined these

couplings and found them to be temper embrittled also. However, the Unit 2

couplings were produced from material with a heat number (heat number 72337)

different from that of the damaged couplings previously found in the Unit 1

pumps.

Discussion

The Brookhaven analysis of the first failed coupling included impact tests on

subsized Charpy V notch samples that were machined from the coupling material.

These samples were about 7.5 mm [0.295 in.] thick and 10 mm [0.394 in.] wide.

Two of these samples were treated at 704�C [1300�F] for 1 hour and oil

quenched. In its report, Brookhaven concluded that the impact energy of the

as‐received material was extremely low (11 to 20 joules [8 to 15 ft‐lbf]) and

was markedly increased (+136 to +163 joules [+100 to +120 ft‐lbf]) after

corrective heat treatment.

The Beaver Valley licensee also reported that Charpy impact tests on material

from the damaged Unit 1 couplings indicated low measured energy absorption

values of 11 joules [8 ft‐lbf] or less using a 7 mm by 10 mm specimen. The

licensee replaced all of the failed and questionable couplings used on the

river water pumps of both units with couplings that had been screened for

toughness and proper heat treatment on the basis of impact properties. The

licensee also modified the purchase specifications for the couplings to

include minimum impact energy requirements for the coupling material. The

licensee issued a 10 CFR Part 21 notification to the NRC on November 1, 1991,

regarding the Unit 1 Byron Jackson river water pump couplings and a

10 CFR Part 21 notification on October 29, 1992, regarding the Unit 2

failures.

The NRC staff understands that Byron Jackson vertical circulator pumps with

coupling configurations similar to those at Beaver Valley are commonly used.

Because couplings manufactured from two different heats of Type 410 stainless

steel have been found with temper embrittlement at Beaver Valley, Byron

Jackson stainless steel couplings at other locations also may have low impact

strength due to temper embrittlement. Pump shafts containing temper.

IN 93‐68

September 1, 1993

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embrittled couplings could fail during operation if the pump has worn
bearings, the pump shaft is misaligned, or shaft motion is impeded by silt or

debris ingestion.

This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

the technical contact listed below or the appropriate Office of Nuclear

Reactor Regulation (NRR) project manager.

/s/'d by BKGrimes

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contact: James A. Davis, NRR

(301) 504‐2713

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